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Lathe Won't Start


Feb 8, 2009
california USA
I had another thread about a thermal relay, but that is not the problem. I'm stumped on what is wrong.


When I engage the spindle lever to forward with the motor off (the motor has a switch (ICS in the schematic below) that selects 3/6 hp, and it also has an off position), the contactors 1M and 2MF energize as is normal. All is good. Everything is energized the way it should be, except no power is reaching the motor since switch ICS, is off.

When I do the same with the motor switch on, the motor comes on for a fraction of a second and then 1M de-engergizes.


I checked phases at the output of 2MF to the motor switch: 225, 235, 240volts.

I bypassed the thermal relay (IOL). No change. As you can see, the only things that can keep 1M from energizing are the emergency cutoff, the door interlock, the thermal relay, and 1M itself once engergized. Also note that 1SAS, iLS, 2LS are part of an electro-magnetic brake are disabled and remain in their normal positions. Once 2MF is energized, 1M must stay energized to keep both relays energized. You can see this in the circuit where the master stop is located. So it seems that 1M has a bad contact. I disassembled 1M and cleaned everything up. Still it drops out.

I next attempt to bypass 1M by forcing it in the engage position. When I do this, 2MF rattles on and off at a very rapid rate. You can see that nothing can de-energize 2MF except the spindle switch (remember that 1SAS stays in the closed position). I can observe 3MR and it is not energizing, which would de-energize 2MF.

To me, this seems to say that the 110volts from the transformer is dropping out low enough to de-energize the contactors when the motor loads the mains. The scenario is that 1M engages, the motor engages, the 110volts drops and then 1M drops.

When I force 1M, the motor starts, the 2MF drops, disconnects the motor, the 110 comes back up, 2MF engages, and it repeats.

I don't have a very sensitive meter, but the motor looks like it is not shorted. So I'm thinking that I have a bad connection that won't support the load when the motor comes on. I've checked everything and it looks good.

Any other ideas?

Power Source?

What is your input power source?

And (out of curiosity) what machine is this? (Reminds me of the print for my Colchester 8017).
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Is this running true three phase or off a rotary converter? Also, post this in the converter/electrical section of the forum. Lots of real smart guys there.
An RPC. It was working fine. The one change I made is that I rewired the line between the RPC and the lathe electrical box. So I have to suspect that is the culprit, but darned if I can find it. All 3 phases and the ground are good solid connections.
What Mike C said. Roll your phases once, if that doesn't fix it roll them again.
Roll the phases = move L1 to L2, L2 to L3, L3 to L1. Keeps the rotation the same while moving the generated leg.
Excellent. Yes, that makes sense. if I rotated the phases during the rewire, that could have changed what's going where. I'll give it a try.

I would have never considered that. Great input. Thanks.
Been there with an RPC on a Bridgeport, UK spec with multi-tap transformer auxiliary feeds for the control gear et al very similar to what you have. Darn thing only works in one specific combination of input phases from the RPC. Ended up having to reverse the transformer feed and control gear supply connections too before I finally got it reliable. Bottom line is that phase shifts are as important as voltages when it comes to driving those multi-tap semi auto transformers. RPC phase angles can wander all over the shop during start up, especially on smaller commercial variants which always use too small a "motor" for the rotary job.

Thought I was gonna have to break out the 'scope to sort that one. If I ever do another one its getting an independent feed to the transformer and master contactor on the 3 phase input side. More work than a straight forward hook up but I know its gonna work first time. I really must figure out the circuits just in case.

L1 to L3 voltage dropping too low

Sounds like the load of the motor is causing the voltage between L1 and L3 to sag, which in turn will lower the 110V feeding your relay coils. When voltage drops on a relay coil it will either drop out as in the case of M1 cutting out after a short time or pull in weakly and bounce on and off as is the case of M2 rattling when you force M1 to stay on.

If resequencing the phases to have the mains leg feed L1 and L3 does not work then you should look for a resistive path in the three phases feeding the motor causing the motor to single phase and draw too much current.
Pipes, yes, I think that is what is going on. I think Mike and Fasto have pointed me in the right direction. I'll try it out later today.
That fixed it. I rewired it so the L1 and L2 from the mains was wired to L1 and L3 in the box (which drives the transformer) and it spun up. Now I just need to remember to put the thermal relay back in.

Great work guys. I don't think I would have ever figured that out.
Glad to be of help. Any time I hear of contactor problems on a three phase machine, my first question is whether it is line 3phase or RPC. If it's RPC 99.999% of the time it will be generated leg on the controls.